Display devices utilising liquid crystal (LC) have historically suffered degraded image quality through loss of contrast ratio as a result of temperature-induced changes in the optical properties of the liquid crystal material. In particular, the voltage-transmission curve of a liquid crystal is related to its temperature, as shown in FIG. 1 of the accompanying drawings.
A well-known solution for this degradation in image quality is to provide a temperature controlled contrast ratio compensation system comprising means for measuring the temperature of the display and means for altering the voltages applied to the display based on this measurement. Such a system is disclosed for a segmented liquid crystal display in EP0012479 and for an AMLCD in U.S. Pat. No. 5,926,162.
Alternatively, a temperature control system may be provided comprising means for measuring the temperature of the display and a heating element to maintain the display at a constant temperature. Such a system is disclosed in JP7230079. In general, systems based on the heating element method are undesirable compared to the driving voltage compensation method due to the increased power consumption associated with the heating element.
Conventional solutions for measuring the temperature rely on attaching a discrete temperature detection element to the display, for example as disclosed in U.S. Pat. No. 5,029,982. Disadvantages of this method include: indirect measurement of the liquid crystal temperature (it is the temperature of the glass, or substrate on which the detection element is mounted, that is actually being measured and not the LC); extra connections to the display reducing reliability; and extra components and fabrication steps raising the cost.
In order to reduce fabrication cost, a liquid crystal temperature sensor may be fabricated with the temperature detection element integrated on the display substrate itself, as disclosed in U.S. Pat. No. 6,414,740. In this disclosure, the temperature detection element is a thin-film diode or thin-film transistor that has a temperature related drain current measured by circuitry separate to the display substrate. Thus the device still has the disadvantages of performing indirect measurement of temperature and requiring extra connections to the display. An additional disadvantage is that the process variation typical of elements integrated onto the display substrate limits the accuracy of such systems.
U.S. Pat. No. 6,333,728 discloses an improved arrangement in which the temperature detection element is formed as a liquid crystal capacitor. The advantage of using a liquid crystal capacitor as the temperature detection element is that it has a one to one transfer function when relating the sensed temperature to the optical performance of the display pixels. The transient response of the liquid crystal capacitor to an input ramp voltage is used as a measure of temperature. In a first embodiment, a differentiator is used to detect the maximum rate of change of this transient response and a peak detection circuit is subsequently used to generate a voltage corresponding to the location of the maximum rate. This voltage is compared with a reference and a heating element is switched on/off according to the relative value. In a second embodiment, a switch arrangement is used to sample the transient response at a defined time. The voltage sampled at this defined time is a function of the capacitance of the liquid crystal element and hence of the temperature. A differential integrator compares the sampled voltage with a reference and its output is used to control the heating element.
In both above embodiments, the system supplies an output voltage corresponding to the difference between a measured temperature-dependant voltage and a reference voltage. Whilst this is suitable for on/off control of a heating element, as in a control loop, disadvantageously the system does not supply a measure of absolute temperature as would be required in a preferred driving voltage compensation system. It is unlikely that this system may be modified to achieve accurate absolute temperature measurements in a practical display system for the following reasons:                the transient response approach to measuring the capacitance of the liquid crystal element requires a ramp input voltage of constant slope. This is difficult to achieve in practice requiring a significant increase in complexity of the display driving circuits;        it is difficult to accurately define capacitor values, including the liquid crystal capacitor element, in practice. Reference voltages and timing signals supplied to the system therefore need to be uniquely calibrated for each display.        